A new species of squeaker frog (genus Arthroleptis) is described from Pico Basile on Bioko Island (Equatorial Guinea), which lies just off the coast of Cameroon in the Gulf of Guinea. A previous molecular phylogenetic study indicates that this new species is nested within a species complex—Arthroleptis poecilonotus—that comprises morphologically indistinguishable populations that represent several cryptic species. The new species from Bioko is, however, easily distinguishable from both closely related and geographically proximate Arthroleptis by a combination of its moderately large body size (approximately 25% larger than frogs in the remaining poecilonotus species complex) and unique ventral color pattern. It is further distinguished from all other Arthroleptis by the presence of spines on the crus in large males and from other populations in the poecilonotus species complex by the occasional presence of a supernumerary tubercle at the base of pedal digit II. Statistical analyses of measurement data, including a principal components analysis, demonstrate that the new species from Bioko is both larger and has relatively longer hindlimbs and toes than other populations of the poecilonotus species complex. These analyses also suggest that the poorly known A. brevipes is morphologically diagnosable, but the status of other taxa such as A. bivittatus and A. zimmeri remains unclear. This new Arthroleptis is the only frog that is unequivocally endemic to Bioko Island.